The present invention relates generally to methods and apparatus for making resistivity measurements in a wellbore, and more particularly relates to such methods and apparatus for making resistivity measurements proximate a drill bit, such as in a logging while drilling or measurement while drilling environment. Such resistivity measurements proximate the drill bit may be used for various types of analysis, including in some instances imaging of the formation and geosteering of the drilling operation.
As is well known, resistivity measurements may be used both to evaluate formation properties, and to identify bed boundaries to assist in steering of a drilling operation. In trying to evaluate formation properties, one problem that is faced is measurement errors resulting from fluid invasion of the drilling mud and associated filtrate into the formation. As this invasion surrounding the wellbore is a function of several factors, including time, it would be desirable to measure resistivity at the drill bit to minimize the time component of the formation invasion.
Because of factors such as the above, the desirability of making resistivity measurements at, or ahead of, the drill bit during a drilling operation has been proposed. However, the actual construction and configuration of a bottom hole assembly, including a drill bit, to facilitate those resistivity measurements presents substantial challenges. These challenges include how to provide electrical communication to and from components in the drill bit, particularly in a configuration that is suitable for the realities of drilling operations, and particularly in systems wherein geosteering mechanisms are employed. Other challenges include how to configure the sensors to obtain the highest quality measurements at the drill bit; how to include the necessary functionality in the drill bit in a manner which is cost-effective; as well as additional challenges. Thus, notwithstanding recognized benefits to making such measurements at the drill bit, limited practical configurations have been proposed to make those measurements.
Accordingly, it would be desirable to have a practical bottom hole assembly, and an included drill bit, that enables the taking of resistivity measurements at or near the drill bit, or ahead of the drill bit, and which addresses at least some portion of one or more of the above challenges.